A DC-DC converter having a charging function, in which a conventional fuel cell, solar cell, or the like is used as an input source, and a load Ro and a secondary battery B are in parallel connected to each other on its output side is shown in FIG. 11. In the DC-DC converter including a DC-DC converter shown in FIG. 11, a means for achieving stabilization of input power by controlling an input voltage to be constant, in which the input voltage is lowered due to an operation of an output impedance in a power-supply mode, has been employed (regarding a solar cell, see, for example, Patent Document 1) in which, when input supply power Pin is less than output supply power Pout, an input voltage is made constant, so that an output voltage enters a drooping state, and in which, when the input supply power Pin is greater than the output supply power Pout, the input voltage rises, so that the output voltage becomes constant.    Patent Document 1: Japanese Laid Open Patent Application Publication No. 11-341699